1. Field of the Invention
The present invention relates to an optical recording medium such as an optical disk or a magneto-optical disk, which has plural recording layers, and a reproduction apparatus and a tracking servo method for the optical recording medium.
2. Description of the Related Art
As an optical recording medium capable of recording or reproducing information optically, an optical disk is widely known. A semiconductor laser or the like is used as a light source to irradiate light beams condensed finely via a lens on the optical disk, whereby information is recorded in the optical disk or reproduced from the optical disk. As publicly known, a focus servo operation is performed in order to keep laser beams in a focused state with respect a recording layer of the optical disk. In addition, tracking servo is performed such that a laser spot can trace a recording track on the optical disk properly in a state in which focus servo is applied to the optical disk.
The focus servo and the tracking servo are performed by moving an object lens, which is held by a biaxial mechanism (a biaxial actuator) in an optical head, in a direction in which the object lens approaches and separates from the optical disk (a focus direction) and a disk radial direction (a tracking direction) on the basis of a focus error signal and a tracking error signal.
As generation systems for the tracking error signal, there are known systems using a push-pull signal obtained from reflected light information such as a push-pull system and a top hold push-pull (TPP) system and a Differential Phase Detection (DPD) system. In addition, there is also known a three-beam system that calculates a difference of sub-beam reflected light of a three-spot laser.
JP-A-2000-20973 discloses a technique concerning the TPP system.